1. Field of the Invention
The present invention relates generally to demodulation of offset quadrature phase shift keyed (OQPSK) signals, also known as staggered quadrature phase shift keyed (SQPSK) signals, and more particularly, to a unified phase and timing detection method and apparatus which facilitate both acquisition and tracking of OQPSK signals.
2. Description of Related Art
Phase shift keying (PSK) describes the modulation technique which uses the input signal to modulate the phase of the carrier. PSK offers a simple way of increasing the number of information symbols in the transmission without increasing the bandwidth.
In the quadrature phase shift keying (QPSK), the carrier phase is modulated to have four possible values, corresponding to the transmitted information symbols 00, 01, 10, 11.
A baseband signal can be expressed as a complex signal having a real part and an imaginary part. The real part is called the in-phase component, the imaginary part is called the quadrature component. In quadrature modulation such as QPSK, two orthogonal carriers cos .omega..sub.c t and -sin .omega..sub.c t are modulated with two independent half-speed data signals which correspond to the in-phase and quadrature components of a baseband signal, respectively, and these modulated carriers are added to form a composite signal to be transmitted. The two carriers have the same frequency but are 90.degree. apart in phase, and define a two-dimensional signal space. The axes in this space are represented by the carriers cos .omega..sub.c t and -sin .omega..sub.c t. The coordinates are represented by the in-phase and quadrature components of the baseband signal. This representation forms a signal constellation.
Three important properties are desirable for data transmission by radio. These are finite bandwidth, zero intersymbol interference (ISI) and constant carrier envelope. All of these properties cannot be achieved absolutely in the same system. Thus, there must be some acceptable compromise with respect to at least one of the properties.
With QPSK, finite bandwidth and zero ISI can be achieved. However, the variation in the carrier envelope of the transmitted signal can be unacceptable. This is caused by the fact that the in-phase and quadrature components can be zero at the same time. If the two component signals are normalized so that the sum of the powers of the two sampled signals is unity, then the continuous sum, which corresponds to the power of the modulated carrier, can vary between a maximum of 1 and a minimum of zero.
However, if the timing of the two component signals is staggered by half the symbol period (or interval), then the two component signals can no longer be zero at the same time. If the two component signals are normalized, then, at sampling times, when one signal is .+-.1/.sqroot.2, the other is either 0 or .+-.1/.sqroot.2. Therefore, the power of the modulated carrier can vary between a maximum of 1 and a minimum of 0.5. This is equivalent to 3 dB of amplitude modulation, which is acceptable for most applications. This modulation technique where the in-phase and quadrature components of the QPSK signal are staggered by half the symbol interval is called offset quadrature phase shift keying (OQPSK), or staggered quadrature phase shift keying (SQPSK). Although an OQPSK signal provides better data transmission properties than a QPSK signal, the OQPSK signal is difficult to acquire and track at the receiver. In other words, it is difficult to recover the baseband timing and the carrier phase of the OQPSK signal.
The prior art approach is to destagger the received in-phase and quadrature components by half a symbol interval and to use standard QPSK approaches to time and phase alignment on the destaggered signals. This approach works well in tracking, since the correct component can be effectively destaggered. However, the destaggering approach does not work well for acquisition of the OQPSK signal for the following reason. Since the phase of the carrier is unknown, one of the in-phase and quadrature channels in the demodulator is blindly delayed. Since the symbols are not yet in phase alignment, this delaying causes severe mixing of nearby symbols and phases. Thus, the acquisition performance of this prior art technique is poor.
Therefore, there is currently a need for a method for efficiently tracking and acquiring an OQPSK signal.